1. Field of the Invention
The present invention relates to a scanning probe microscope and a method of using the same.
2. Description of the Background Art
A scanning probe microscope (SPM) includes a scanning tunneling microscope (STM) and an atomic force microscope (AFM). The STM measures tunneling current flowing between a sample and a conductive probe and scans a region to be observed, thus forming an image of the region. The AFM measures atomic force interaction between the tip of a probe and a sample surface and scans a region to be observed, thus forming an image of the region.
According to the AFM, atomic force interaction between the tip of a probe fixed to the end of a plate spring (that is a so-called cantilever) and a sample surface is measured by the amount of deflection (displacement) of the cantilever. A sample surface is scanned by the cantilever. The image of the shape of the sample surface is thereby formed.
Several methods are known in AFM that are classified according to how displacement of a cantilever is detected. More particularly, they include an advanced STM method, an optical lever method, an optical interaction method and the like. According to the advanced STM method, a cantilever is arranged between a conductive probe of the STM and a conductive sample. Displacement of the cantilever is detected as a variation in tunneling current. According to the optical lever method, displacement of a cantilever is detected by a variation in angle of laser light reflected at the back of the cantilever. According to the optical interaction method, displacement of a cantilever is detected by measuring the amount of interaction between light reflected at the end face of an optical fiber arranged on the back of the cantilever and light reflected at the back of the cantilever. Among these, the optical lever is a dominating method in current AFMs.
FIG. 12 is a view illustrating the structure of an optical lever AFM. The AFM in FIG. 12 is an electron microscope allowing, in addition to observation of the shape of a sample surface, measurement of the sample by electrical characteristic.
With reference to FIG. 12, reference numeral 1 designates a laser diode, numeral 2 designates laser light emitted from the laser diode 1 and numeral 3 designates reflected laser light obtained by reflecting the light 2. Further, a reference numeral 4 designates a photodetector for detecting variation in angle of the reflected laser light 3 and numeral 5 designates a conductive cantilever for reflecting the laser light 2 at the back thereof. Still further, reference numeral 6 designates a piezoelectric actuator for moving a below-described stage mounted on the upper surface thereof in X, Y and Z directions.
Reference numeral 7 designates a controller for forming an AFM image and controlling the piezoelectric actuator 6, numeral 8 designates an amplifier for detecting and amplifying a value of current flowing between a sample and the conductive cantilever 5. Yet further, reference numeral 9 designates a probe fixed to the end of the conductive cantilever 5, numeral 10 designates a stage for holding the sample to be evaluated and numeral 11 designates a variable voltage source for applying voltage to the sample during measurement of the sample by electrical characteristic.
Using variation in angle of the reflected laser light 3 detected by the photodetector 4, the controller 7 calculates the amount of deflection of the conductive cantilever 5 and forms an AFM image. Namely, an image of a shape of the sample surface is formed. Further, the controller 7 controls the piezoelectric actuator 6 in feedback control in such a manner that the amount of deflection of the conductive cantilever 5 is always kept at a certain level. Formation of the AFM image and feedback control will be discussed in more detail as follows. That is, the piezoelectric actuator 6 is controlled to be displaced in a Z axis direction (vertical direction) such that the amount of deflection of the conductive cantilever 5 is kept at a certain level. The amount of surface roughness is detected by the amount of deflection of the actuator 6, thus allowing formation of the AFM image.
In addition to detection of atomic force interaction with the conductive cantilever 5 and the sample, the conductive cantilever 5 further detects the sample by electrical characteristic. Regarding the probe 9, in addition to detection of atomic force interaction with the sample, it also measures the sample by electrical characteristic as a conductive probe contacting the sample. Utilizing a conductive cantilever, surface roughness and electrical characteristic of the sample can be simultaneously measured.
For measurement of the sample by electrical characteristic, voltage is applied to the sample from the variable voltage source 11. A value of current flowing between the sample and the conductive cantilever 5 is detected and outputted by the amplifier 8.
When it is required to detect a defective point in a semiconductor device where current leakage occurs, for example, observing an image obtained by a microscope, leakage current is simultaneously measured in situ. The defective position suffering from leakage current is thereby identified on nanoscale and a current differential between the defective position and a normal position is determined, thus yielding a result such as estimation of the cause of the defect.
The AFM in FIG. 12 may experience leakage of the laser light 2 for measuring displacement of the conductive cantilever 5, which then impinges on the sample. In other cases, a portion of the reflected light 3 may undergo further reflection, thereafter impinging on the sample. Supposing that the sample is a semiconductor, photoelectric current may occur inside the sample in these cases, thus exerting influence on measurement of the sample by electrical characteristic. Such influence is troublesome especially when leakage current of minute amount generated in a semiconductor device should be measured. In this case, a value of leakage current cannot be measured with precision.
The foregoing problem is described according to a photoelectric current generation model in FIG. 13. With reference to FIG. 13, reference numeral 12 designates a semiconductor as a sample and numeral 13 designates an electron-hole pair generated due to leakage of the laser light 2 from the conductive cantilever 5. The semiconductor 12 includes a p-type semiconductor substrate holding an n-type region 12a formed therein, for example. For the sake of simplification, the stage 10 is omitted from FIG. 13.
When a depletion layer 12b created at a pn junction between the n-type region 12a and the p-type substrate is irradiated with the laser light 2, the electron-hole pair 13 is generated as illustrated in FIG. 13, thus generating photoelectric current. As described, photoelectric current thereby generated inhibits precise measurement of current developing inside the semiconductor device.
It is therefore an object of the present invention to provide a scanning probe microscope and a method of using the same preventing generation of photoelectric current in a sample due to laser light.
According to a first aspect of the present invention, the scanning probe microscope has a light source and a cantilever for detecting atomic force interaction with a sample. The scanning probe microscope induces light emitted from the light source to impinge on the cantilever and detects reflected light obtained therefrom, thus forming an image of a shape of a sample surface. The cantilever also serves as a probe for measuring the sample by electrical characteristic. The sample is a semiconductor. The light emitted from the light source has a smaller amount of energy than the band gap of the semiconductor.
In the scanning probe microscope according to the present invention, the light source emits light having a smaller amount of energy than the band gap of the semiconductor. It is thus allowed to measure the sample by electrical characteristic and observe the shape of the sample surface simultaneously without generating photoelectric current in the semiconductor as a sample and eventually, without obstructing precise measurement of the sample by electrical characteristic.
Preferably, the sample includes silicon and the light emitted from the light source is of a wavelength larger in value than 1.107 xcexcm.
In the scanning probe microscope according to the present invention, the light emitted from the light source is larger in value than 1.107 xcexcm. It is thus allowed to avoid emission of light from the light source having a larger amount of energy than the band gap of silicon as a sample and eventually, generation of photoelectric current in the sample.
According to a second aspect of the present invention, the scanning probe microscope has a light source and a cantilever for detecting atomic force interaction with a sample. The scanning probe microscope induces light emitted from the light source to impinge on the cantilever and detects reflected light obtained therefrom, thus forming an image of a shape of a sample surface. The cantilever also serves as a probe for measuring the sample by electrical characteristic. The scanning probe microscope further has a light blocker for blocking the light emitted from the light sources, thus preventing the light from impinging on the cantilever during measurement of the sample by electrical characteristic.
The scanning probe microscope according to the present invention further has the light blocker for blocking the light emitted from the light source, thus preventing the light from impinging on the cantilever during measurement of the sample by electrical characteristic. As a result, there will be no photoelectric current to be generated in the sample, thus causing no obstruction to precise measurement of the sample by electrical characteristic.
According to a third aspect of the present invention, the scanning probe microscope has a probe and a cantilever for detecting atomic force interaction with a sample. The scanning probe microscope detects tunneling current flowing between the probe and the cantilever, thus forming an image of a shape of a sample surface. The cantilever also serves as a probe for measuring the sample by electrical characteristic. The cantilever includes an insulating layer defined between a side facing the probe and a side facing the sample.
The scanning probe microscope according to the present invention has no light source. Therefore, there will be no photoelectric current to be generated in the sample, thus causing no obstruction to precise measurement thereof by electrical characteristic. Further, the cantilever includes the insulating layer defined between the side facing the probe and the side facing the sample. As a result, tunneling current flowing between the probe and the cantilever will exert no influence on measurement of the sample by electrical characteristic.
These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.